A major advance in the field of aortic surgery in recent years is the use of stent grafts for the treatment of aortic diseases. Such use generally involves: delivering a stent graft to the aortic lesion with a dedicated delivery system; and then causing the expansion of the stent. Suitable indications of stent grafts include aortic dissections, true aortic aneurysms, false aortic aneurysms, penetrating aortic ulcers, etc. This technology separates aneurysms from the normal blood flow and thereby eliminates the risk of death from hemorrhage caused by ruptured aneurysms or from compression of surrounding tissues and organs by aneurysms, thus greatly reducing operative mortality, postoperative complications, surgical trauma and patient recovery time. Examples of commercially available stent grafts include those marketed under the brands Valiant, Zenith, Relay, Gore, Hercules and Ankura.
However, these commercially available stent grafts are only suitable for the treatment of descending thoracic aortic aneurysms with a 15-mm long normal thoracic aortic aneurysmal wall from the aneurysm or dissection tear to the left subclavian artery. Currently, there is no stent graft available for patients with aortic dissections or aneurysms involving the ascending aorta and the aortic arch, and the patients have to resort to the conventional surgical approach or the chimney technology, as shown in FIGS. 1a and 1b. 
With the development of minimally invasive technologies, some branched stent grafts have been proposed. From a structural point of view, such branched stent grafts can not only be used to treat the diseases involving the ascending aorta and aortic arch, but also to replace those currently applied in the practical use for treating descending aortic diseases. They enable more safe and reliable treatment by overcoming some disadvantages of the existing stent grafts and by reducing surgical complications.
Chinese Utility Model Publication No. CN201020524397 disclosed a thoracic aortic stent structure including a main vascular prosthesis and one to three branches disposed on the main vascular prosthesis. Chinese Utility Model Publication No. CN200920218598 disclosed an aortic endoluminal stent mainly for use in repair of vascular diseases. The aortic endoluminal stent is essentially consisted of a main body, which is covered with a cover or a braid layer, and a side branch. The main body is composed of a plurality of sections which are interconnected to one another in such a manner that the main body has a curved contour capable of facilitating the repair of a branched portion of a curved vessel. U.S. Pat. No. 7,914,572 disclosed a stent graft with a side branch, in which a main stent graft has a fenestration to which the side branch is so stitched that it can partially extend into the main stent graft. U.S. Pat. No. 7,314,483 disclosed a stent graft with a branch leg, wherein the branch leg is anchored to a main stent graft.
A branched stent graft is typically fabricated in the following manner. At first, a metal material (e.g., NiTi wires or stainless steel wires) is used to produce stents, each of which is subsequently stitched onto a cover (e.g., made of polytetrafluoroethylene, nylon, polyester, terylene or polypropylene) using a polymeric suture (e.g., made of polytetrafluoroethylene, nylon, polyester, terylene or polypropylene), so as to respectively form a main body 10 and a side branch 20 of the stent graft, as shown in FIG. 2a. The main body 10 and the side branch 20 are then stitched together, as shown in FIG. 2b. Typical dimensions of a branched stent graft include: the main body 10 has a length of 60-210 mm and a diameter of 24-50 mm, and the side branch 20 has a length of 10-50 mm and a diameter of 6-18 mm. Further, the side branch 20 should be adjoined to the main body 10 in such a way that a length L of the cover is reserved at the joint to ensure that the assembled branched stent graft can be crimped into an outer sheath 4 of a delivery system 1, as shown in FIG. 2b. 
With the main body 10 and side branch 20 have been assembled, they are inserted into the outer sheath 4 of the delivery system 1. In this process, the main body 10 is first contracted to a diameter similar to that of the outer sheath 4 by using contraction lines 30. The side branch 20 is then received in a branch sheath 7 and folded together with the branch sheath 7 towards a proximal end of the main body 10 such that the side branch 20 is parallel to the main body 10. In this configuration, the whole branched stent graft can be positioned inside the sheath 4, as shown in FIG. 3b. Each stent section of the side branch 20 is stitched to the cover in a manner like the section B shown in FIG. 5a, i.e., being entirely stitched from end to end. After this process, a distance L from the side branch 20 to the main body 10 must be ensured to at least exceed a radius of the side branch 20. Otherwise, the side branch 20 cannot be folded to an orientation that is parallel to the main body 10 and thus cannot be received in the outer sheath 4.
However, currently, no assessment has ever been conducted to evaluate the safety and efficacy of these conventional branched stent grafts. In particular, as noted above, in order to facilitate the assembly and deployment, a length L of several millimeters (generally greater than 3 mm) of the cover, on which no stent section is stitched, is arranged between the side branch 20 and the main body 10. One reason for this design is that, if the side branch 20 and the main body 10 are stitched together without the cover being reserved therebetween, in the assembled state, it is impossible to configure the side branch 20 to make it parallel to the main body 10, thus failing the positioning of the side branch 20 in the outer sheath 4 of the delivery system 1. Therefore, all the existing branched stent grafts have such a cover section with length L between the main body and the side branch. That is, the main body and the side branch are interconnected only by the flexible cover section not being supported by any stent section. This tends to cause narrowing of the side branch at its base portion, as well as an occlusion thereof in case of inaccurate positioning in the deployment process. Further, even after such a stent has been successfully deployed, it is likely that endothelialization occurs and fills the gap between the cover section and the vessel wall and thus causes vascular stenosis or occlusion.